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Charge Pump Frequency
« on: September 05, 2007, 10:42:40 PM »
I turned on a charge pump signal and output it to an oscilloscope.  The frequency I got is half of the 12.5kHz.  Is this correct and just a different definition of frequency?  The frequency of transitions voltage transitions is 12.5kHz.

Re: Charge Pump Frequency
« Reply #1 on: September 06, 2007, 08:23:57 AM »
If the pic you show is the measurement of your charge pump then it is showing 12.5 kHz correctly. One complete cycle includes a high period (as you have marked between the lines) and a low period. The frequency (f) on the right measures 12.5kHz


I hope this clarifys measuring frequency and is helpful.  ;D

Kind Regards
Re: Charge Pump Frequency
« Reply #2 on: September 06, 2007, 01:33:49 PM »
One cycle is from "say" the start of the high transition to the end of the low transition i.e. "one cycle"

Therefore the frequency shown on the scope is approx 6.5KHz - As shown by the built in frequency meter.

Phil.
« Last Edit: September 06, 2007, 01:37:09 PM by PhilW »

Offline Whacko

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Re: Charge Pump Frequency
« Reply #3 on: September 06, 2007, 01:48:12 PM »
Have you checked if the "charge pump frequence" tick box for 5khz is checked?

Whacko
Nothing's impossible
Re: Charge Pump Frequency
« Reply #4 on: September 06, 2007, 02:29:49 PM »
The 12.5kHz shown on the right side is for the cursors.  i manually set them at 12.5kHz for reference.  The actual frequency is the 6.49kHz as shown below that.  The 5kHz for 'Laser Standby' is not checked.  Is Mach toggling the pin at 12.5kHz?  I checked it on Mach2 and got the same frequency.

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Re: Charge Pump Frequency
« Reply #5 on: September 07, 2007, 01:20:02 PM »
Excuse my curiosity, but do you need the frequency to be accurate?

Whacko
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Re: Charge Pump Frequency
« Reply #6 on: September 07, 2007, 01:53:18 PM »
I don't need it accurate.  It was falling below the threshold of my detection circuit.  I can lower the threshold easily.  I was just hoping someone clarify the issue.

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Re: Charge Pump Frequency
« Reply #7 on: September 07, 2007, 01:58:22 PM »
Its very easy if you use a capacitor decoupling circuit. Or driving a small signal transformer. The good part of it, it's tolerant of a huge frequency discrepancy.

Whacko
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Offline Jeff_Birt

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Re: Charge Pump Frequency
« Reply #8 on: September 10, 2007, 09:43:10 AM »
I'm kind of curious about this issue as well.  It seems the charge pump signal should be just as accurate as the axis step signals.  To actually be of use the charge pump signal should be fairly accurate, say 12.5 +- 0.5 kHz.  (I think that is pretty loose, but still OK).  If Mach can't maintain the accuracy of the charge pump where is the guarantee that the axis signals will accurate?

I have not noticed the change in freq. as the OP.  I built my own CP circuit on my break-out board too so I have spent some time hanging a scope on the signal and purposely tried to give it a narrow band of operation.

To the OP:  Have you been watching your axis step signals at the same time?  Are they also slow when the CP is slow?  Does your driver test appear to be OK?
Happy machining , Jeff Birt
 

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Re: Charge Pump Frequency
« Reply #9 on: September 10, 2007, 05:13:39 PM »
I think the consistency of the timing of the step pulses is only important if you are in cv mode! I have scoped a AMD Athlon 64 and a INTEL Celeron 2.4 now, both are on winxp pro, and I must say, that 12,5 khz signal was pretty constant and precise. On the Dell laptop it is a nightmare.

Whacko

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